The present invention relates to an electrical measuring and testing device and method and more particularly to a device and method for precisely and remotely determining the speed of a single-phase or polyphase induction motor.
Electrical motors play a key role in industry, providing the driving torque for fans, pumps, compressors, valves, and many other machines. It is potentially very costly to allow a significant problem to go on unnoticed in the motor, just as it is costly to attempt to repair a nonexistent problem. A parameter that must be known for field evaluation of motors is motor shaft speed or equivalently slip, where slip and shaft speed are related by the expression:
Slip=1xe2x88x92(Shaft Speed/Synchronous Speed)
For example, motor shaft speed must be known in order to determine motor efficiency. Thus the accurate measurement of motor shaft speed is very important for on-line evaluation of motor condition.
Typically a tachometer is used for field measurement of motor shaft speed. One implementation of a tachometer monitors the response of a magnetic pickup or an eddy current probe to a shaft keyway as the keyway passes the pickup or probe. Another implementation senses the reflection of a light beam off of a piece of reflective tape bonded to a shaft. Yet another implementation employs a coil to sense a magnet attached to a shaft. In any of these methods a signal is generated each time the shaft revolves, the resulting signal frequency being the frequency of shaft revolution. There are several problems with these methods, e.g.: a) the motor must be stopped in order to bond the magnet or reflective tape to the shaft; b) attached magnets can become loose over time; c) light reflectors become dirty, disrupting the optical signal; d) in many cases the motor shaft is not easily accessible, having a very short exposed surface or covered by a protective housing; e) in cases where the shaft is accessible, the placement of the eddy current or optical probe can be problematic, getting in the way of routine inspections and maintenance; f) eddy current, optical and other externally added probes jut out from the natural contour of the machine, becoming exposed to the constant risk of being knocked out of alignment (and thus operation) during routine inspection and maintenance; and g) a tachometer requires running cables from the motor back to a central monitoring location, typically the MCC (Motor Control Center).
The ideal speed monitor is non-intrusive, accurate, reliable, capable of remote operation, and not dependent on attachments to the shaft or probes which jut out from the machine. Furthermore, speed monitor sensors should be relatively inexpensive and not require calibration.
The referenced invention disclosed in U.S. Pat. No. 6,144,924 overcame most of the aforementioned problems by determining shaft speed from an analysis of the motor current signal alone. Subsequent testing has revealed that the referenced invention does not accurately detect shaft speed for some conditions. This new invention refines the technique of shaft speed determination by taking into account both motor voltage and current information.
Briefly stated the motor speed monitor comprises a method of determining a shaft speed of a motor by using an electrical signature of the motor. The method comprises: (a) sensing an electrical current supplied to the motor to generate a current sensor output signal for at least one electrical phase of the motor; (b) demodulating the current sensor output signal for a predetermined time interval to obtain an instantaneous amplitude of the electrical current signal; (c) generating a frequency spectrum of the instantaneous current amplitude; (d) finding at least one spectral peak of the instantaneous current amplitude frequency spectrum within a predetermined shaft frequency sideband range; (e) estimating a shaft frequency from the at least one spectral peak; and (f) converting the shaft frequency to shaft speed.
Another aspect of the motor speed monitor comprises a method of determining a shaft speed of a motor by using an electrical signature of the motor. The method comprises: (a) sensing an electrical voltage supplied to the motor to generate a voltage sensor output signal for at least one electrical phase of the motor; (b) sensing an electrical current supplied to the motor to generate a current sensor output signal for at least one electrical phase of the motor; (c) demodulating the voltage sensor output signal for a predetermined time interval to obtain an instantaneous phase of the voltage sensor output signal; (d) demodulating the current sensor output signal for a predetermined time interval to obtain an instantaneous phase of the current sensor output signal; (e) subtracting the instantaneous phase of the current sensor output signal from the instantaneous phase of the voltage sensor output signal to obtain an instantaneous difference angle; (f) generating a frequency spectrum of the instantaneous difference angle; (g) finding at least one frequency peak of the instantaneous difference angle frequency spectrum within a predetermined pole pass frequency sideband range; (h) estimating a pole pass frequency from at least one spectral peak; and (i) converting the pole pass frequency to shaft speed.
A further aspect of the motor speed monitor comprises a method of determining a shaft speed of a motor by using an electrical signature of the motor. The method comprises the steps of: (a) estimating a shaft frequency by measuring at least one first spectral peak location in a first frequency spectrum of an amplitude demodulated motor electrical current; (b) estimating a pole pass frequency by measuring at least one second spectral peak location in a second frequency spectrum of the difference between a phase demodulated motor electrical current and a phase demodulated motor electrical voltage; (c) measuring the consistency of the shaft frequency and the pole pass frequency estimates respectively by comparing the location of the first and second spectral peaks in at least one motor phase; (d) calculating and outputting the shaft speed based on the most consistent of the shaft frequency and the pole pass frequency estimates; (e) calculating and outputting the shaft speed as the average of a first shaft speed calculated from the shaft frequency estimate and a second shaft speed calculated from the pole pass frequency estimate if the shaft speed estimate and the pole pass frequency estimate are equally consistent and if the first shaft speed differs from the second shaft speed by less than a predetermined limit; and (f) not outputting the shaft speed if the shaft frequency and the pole pass frequency are equally consistent and if the first shaft speed differs from the second shaft speed by a value equal to or greater than a predetermined limit.